An image forming apparatus such as a copy machine or printer has a developing unit that casts a laser beam onto a charged photoconductor and applies a developer to a site where electric potential is changed. To maintain the electric potential of the photoconductor at a constant level, a surface potential sensor that measures the potential of the photoconductor is provided.
Conventionally, the surface potential sensor is installed upstream from a developing section, which is the position coated with the developer by the developing unit, in the direction of rotation of the photoconductor. Therefore, as a urethane seal in weak contact with the photoconductor is arranged between the surface potential sensor and the developing section, a floating developer can be prevented from attaching to the surface potential sensor.
However, if the laser unit is miniaturized in order to miniaturize the image forming apparatus, a long optical path length cannot be provided for the laser. Consequently, the surface potential sensor must be installed downstream from the developing section in the direction of rotation of the photoconductor.
If the surface potential sensor is installed downstream from the developing section in the direction of rotation of the photoconductor, the urethane seal cannot be arranged in weak contact with the photoconductor since the photoconductor is already coated with the developer at the position of the installation.
With respect to this point, a technique of providing a suction duct that collects a floating developer by a negative pressure, downstream from a developing section of an image forming apparatus in the direction of rotation of a photoconductor, is proposed (see, for example, JP-A-2003-29522) Also, a technique of blowing air from a blow duct and thus preventing floating toner from attaching to a surface potential sensor is proposed (see, for example, JP-A-4-56975).
However, if the duct is miniaturized in order to further miniaturize the image forming apparatus, places having strong and weak air flows are generated. This causes a problem that the floating developer cannot be collected efficiently.